Two-and-a-half starsIt’s no secret that little kids love talking animals. But is it too much to ask that there be a stated reason why a koala, elephant, porcupine and pig live happily in the same urban city and all have a passion for music?That’s the crux of the so-so “Sing” movie series — tales of a showbiz zoo in which someone’s status as a turtle is hardly ever mentioned.